1) you got an external, USB SSD device and got this formatted to FAT32 using (im-magic-formater)
2) you used the Brennan to import 4500 songs from the external, USB SSD which you connected to a USB port on the rear of the Brennan. AND that this worked OK.
3) the Brennan is now saying it is compressing to FLAC.
If the above is correct then the Brennan is compressing (to FLAC format) the 4500 files/songs that it/you copied - (from the external, USB SSD device connected to a USB port on its rear ) - onto its internal disk (HDD).
Leave it to finish this process (yes it is slow)
The files on the SSD (that will still be there) will not be changed/compressed by the Brennan, it ONLY compresses the content on its internal disk.
Fred